1. Field of the Invention
The present invention relates to a write-data signal compensation apparatus for disk storage used in a fixed disk apparatus or the like, and in particular, to a compensation apparatus for modifying a first write signal having a particular modulation data pattern into a second write signal having a data pattern expedient to be read out from a disk before the second write signal is written to the disk.
2. Description of the Prior Art
In disk storage, data are written in a number of tracks on a disk in a particular modulation pattern. These patterns are similar to digital data in that they are composed of an alternate pattern of N and S poles, i.e., "0" and "1". However, the patterns differ from the plain digital data in that they are recorded in a pattern modulated by a modulation technique such as MFM (Modified Frequency Modulation) or RLL (Run-Length Limited code) system. As a result, transitions from "0" to "1" or "1" to "0" in the pattern, and the length therebetween have a significant meaning. These patterns, when read out, give a differential waveform of the original pattern. This means that the read out waveform has maximal and minimal peaks at the transitions from "0" to "1" or "1" to "0", and hence, the original pattern can be reproduced by detecting these peaks, and thus, the write data can be decoded.
The transitions from "1" to "0" or vice versa in the read signal have definite intervals when the peak points are correctly detected: in one example of the MFM system, two intervals of 200 ns and 300 ns appear in the read signal, and this makes it possible to decode the original data.
In practical disk storage, however, the circumferential velocity of inner tracks on a disk is slower than that of outer tracks. Consequently, the level of the read signal from the inner tracks is significantly lower than the level from the outer tracks. In addition, the record density of the data pattern is higher in the inner tracks than in the outer tracks. This will cause a difference in the waveforms of the read signal from the inner tracks and the outer tracks. To align the level of the read signals from the inner and outer tracks, an AGC (Automatic Gain Control) amplifier is used. However, this tends to cause a "peak shift" which means that the peak positions detected are displaced from the correct positions. This is because the waveform difference between the read signals from the inner tracks and the outer tracks is enlarged by the frequency characteristics of the amplifier because patterns of different periods are mixed together in the read signals. Thus, the correct reproduction of the original data patterns becomes difficult, and the occurrence of read errors tend to increase.
The write signal compensation which is the object of the present invention is provided to correct the peak shift. Although the occurrence mechanism of the peak shift is significantly complicated, it has a certain inclination in accordance with data patterns. Consequently, the data pattern to be written on a disk can be compensated by shifting parts of the data pattern corresponding to the positions of peak shift in the read signal in advance by the amount inferred from the read signals in the direction reverse to that of the peak shift.
Conventionally, such a compensation of the write signal is achieved by a controlling portion incorporated or combined with disk storage. As is known, the controller includes a processor and a signal processing circuit, and is connected to a host CPU through buses and interfaces.
Locations on a disk to which data are written are designated from the host CPU to the controller in the form of logical addresses in the memory space of the disk storage. The processor in the controller then converts the logical addresses to physical addresses and instructs them to the disk storage.
The data to be written to the disk are also transferred from the CPU to the controller. The data processing circuit in the controller converts the data into the write signal according to a predetermined modulation scheme, and transfers it to the disk storage.
The controller is originally provided with a function for converting the write data into the write signal, and so the compensated write signal can be easily produced by slightly modifying the converting function. Furthermore, the controller is also provided with a function to calculate the track number, and hence, the compensation for the write signal can be easily performed corresponding to the write addresses of the data.
Producing the compensated write signal instead of an ordinary write signal in the controller makes it possible for the disk storage to simply receive the signal and write it on the disk. Thus, when the compensation is performed appropriately, the original data pattern can be reproduced with high fidelity and read errors of the data can be prevented, even if the peak shift errors occur during the peak detection of the read signal.
As described above, the conventional controller can rather easily compensate the write signal. However, increasing peak shifts will occur with the increase in the memory capacity of recent disk storage. This makes it difficult for the controller to compensate the write signal with high accuracy.
The waveform of the read signal of a data pattern is greatly affected by the following features of the disk storage: the type of a memory medium of a disk, the frequency characteristics of the AGC circuit for the read signal, the construction of the peak detection circuit, as well as the pattern recording density. However, the controllers are generally arranged to fit better to the CPU than to the disk storage in order to meet the requirement of the CPU. In addition, the controllers are designed so that they can be combined with any ordinary disk storage. Thus, it is difficult to precisely compensate the write signal to fit to any special features of the disk storage.
Theoretically, it is possible for the controller incorporated in disk storage to meet any special features of the disk. However, it becomes increasingly difficult to accomplish the precise compensation required by the recent high density recording of the data pattern: the controller, which is generally restricted in its capability and performance, can hardly achieve the precise compensation by switching compensation amounts in accordance with the radial position of a head.
Furthermore, many controllers are not provided with the write data compensation function at all. In such a case, improvement in reliability of the read operation of the disk storage, and thereby increasing the memory capacity must be given up.